Variable-speed, constant-frequency (VSCF) systems and other high power airborne applications require the use of high voltage capacitors in small packages to filter currents produced by the systems. Ceramic capacitors offer several advantages over other types when used in conjunction with high power electrical devices. In fact, large multilayered ceramic capacitor assemblies can be used to replace several smaller capacitors wired in parallel. These ceramic capacitors are rectangular in shape and have a width and height typically in excess of an inch and a depth of somewhat less than an inch. These capacitors include a plurality of ceramic layers separated by intervening conductive coatings. A portion of the coatings are connected together at one edge of the capacitor by a conductive electrode. The remaining layers of conductive coatings are connected together at a second edge of the capacitor by a conductive electrode.
Mounting of these large ceramic capacitors to a printed circuit board, chassis wall or other planar surface has proven to be a difficult task in the past. Because of their size, the capacitors must be mechanically fixed to the planar surface. Furthermore, due to the high quantity of power passing through the capacitors, significant electrical contact must be made to the capacitor electrodes.
Mounting was done typically in two steps. First, the body of the capacitor was usually mechanically fixed to the planar surface by cement or by pooling solder on the planar surface and pressing the dielectric body of the capacitor into the solder as it hardens. Second, electrical contact was accomplished by bringing electrical leads from the planar surface to the electrodes on the capacitor. Accordingly, mounting of large ceramic capacitors was accomplished by trial and error and had to be repeated individually for each capacitor to be mounted.
Apart from an unsightly mess on the printed circuit board and problems resulting from the pooled solder creating undesired electrical paths on the board, a problem encountered with the prior mounting methods was that significant thermal expansion of the capacitor would occur during operation. Accordingly, extreme stress would be imparted to the cement or solder holding the body of the capacitor to the planar surface. Occasionally, the cement or solder would loosen, allowing the capacitor to separate from the planar surface. At other times, the cement or solder would not allow the capacitor to expand, in turn resulting in catastrophic failure.
Finally, the prior mounting method failed to provide a means by which the capacitor could be cooled during operation, thereby relieving stress on the capacitors and lengthening their lives.
For years, low power applications have involved the mounting of capacitors to printed circuit boards. However, low power applications do not have to address the mechanical mounting, power handling or thermal expansion problems encountered in high power applications.
Harper et al., U.S. Pat. No. 4,401,843 discloses various structures for, and methods of, constructing miniaturized high capacitance bus bars. The bus bars incorporate discrete capacitive elements between a pair of bus bar conductors. Alternative arrangements are presented wherein the elements are maintained in electrical contact with the conductors while avoiding the development of short circuits between the two bus bar conductors.
Harper et al. U.S. Pat. No. 4,451,694 is directed to various structures for, and methods of constructing, miniaturized high capacitance bus bars. The bus bars incorporate discrete capacitive elements between a pair of bus bar conductors. In a preferred embodiment presented in this divisional of the Harper et al. '843 patent, a zebra film comprised of an elastomeric material having alternating strips of conducting and non-conducting material is positioned between the capacitive elements and the bus bar conductors. This zebra film establishes electrical contact between the capacitive elements while avoiding the development of short circuits between the two bus bar conductors. The zebra film may also have a mechanical force means applied whereby the applicable conductive surfaces are urged into electrical contact with the elastomeric strip material.
Erdle, U.S. Pat. No. 4,517,406 discloses one or more multilayer ceramic capacitors positioned in a laminated bus bar so that a pair of opposed, external electrodes on each capacitor are positioned in substantially coplanar engagement with the confronting plane surfaces of adjacent conductor strips in the bar. The two external electrodes of each capacitor are connected each to a different one of two spaced metal termination plates that are mounted on each capacitor. The two termination plates of each capacitor register with spaced recesses or openings formed in each the adjacent conductor strips to accommodate any projections on the termination plates and thereby permit coplanar engagement of the external electrodes with the conductor strips.
Herrandez, U.S. Pat. No. 4,599,486 discloses a miniaturized surface mountable bus bar wherein a sheet of insulating material is laminated between a pair of bus conductors and windows are provided in the laminated structure. A plurality of multilayer ceramic capacitor elements are inserted in the windows and alternate conductive side plates of the capacitors are electrically connected to the two bus conductors. The capacitors are comprised of alternating layers of conductive material and dielectric material having opposed terminating side conductors which are oriented parallel to the sheet of insulating material and the pair of bus conductors after assembly thereof.